Multi-internal standard calibration applied to inductively coupled plasma optical emission spectrometry

Several species are simultaneously used for internal standardization to improve accuracy in inductively coupled plasma optical emission spectrometry (ICP-OES). In multi-internal standard calibration (MISC), signal ratios between the analyte and each of several internal standard species are used for calibration. A single calibration solution is required, and the MISC graph is built with intensity ratios calculated with analytical signals recorded for the sample (I_A,sam) on the y-axis, while those recorded for the calibration standard (I_A,std) are plot on the x-axis (i.e. I_A,sam/I_IS(i)vs. I_A,std/I_IS(i), where I_IS(i) represents the signal intensity for a given internal standard species). Nine analytes (As, Cd, Cr, Cu, Fe, Mg, Mn, Pb and Zn), and two sets of internal standard species (i.e. Bi, Ge, In, Rh, Sc, Te, Tl and Y in solution, or eighteen emission lines from plasma naturally occurring Ar) were evaluated in this proof-of-concept study. The MISC method's efficiency was evaluated by analyte addition and recovery experiments and by analyzing two certified reference materials. Figures of merit for MISC (limit of detection, repeatability and trueness) were comparable to those obtained for the traditional external standard calibration (EC) and internal standard (IS) methods. Different from IS, MISC requires no time-consuming study to identify an ideal internal standard species, and signal biases are minimized by an averaged, more encompassing internal standardization effect.